Ksp calculator for solubility and precipitation (Q vs Ksp)
The solubility product Ksp is the equilibrium constant for dissolving a sparingly soluble ionic solid. For a salt that dissolves as x cations and y anions per formula unit, Ksp = [cation]^x[anion]^y at saturation. Molar solubility s is the moles of formula unit that dissolve per liter of saturated solution in pure water.
To calculate Ksp from solubility, write the dissolution equation, express each ion as a multiple of s, and multiply. Example: AgCl (type AB) with s = 1.34×10⁻⁵ M gives Ksp = s² ≈ 1.8×10⁻¹⁰. To find Ksp the other way, invert the same relation: s = √Ksp for AB, or s = (Ksp/4)^(1/3) for AB₂ / A₂B.
Comparing the ion product Q (same form as Ksp but with actual concentrations) to Ksp predicts precipitation: Q > Ksp favors solid formation, Q < Ksp means the solution is unsaturated. The Ksp Calculator converts between s and Ksp for common salt stoichiometries and evaluates Q versus Ksp as a precipitation reaction check.
- AB salt: Ksp = s²
- AB₂ salt: Ksp = 4s³
- A₂B salt: Ksp = 4s³
- Q > Ksp → precipitate expected
Formulas you will actually use
For MxAy(s) ⇌ x M + y A with solubility s:
Ksp = (x s)^x (y s)^y = x^x y^y s^(x+y)
s = (Ksp / (x^x y^y))^(1/(x+y))
Q = [M]^x [A]^yStep-by-step example: Solubility of AgCl from Ksp
AgCl is type AB with Ksp = 1.8×10⁻¹⁰ at 25 °C. Find the molar solubility in pure water.
- Ksp = s² = 1.8×10⁻¹⁰.
- s = √(1.8×10⁻¹⁰) ≈ 1.34×10⁻⁵ mol/L.
Choose Ksp → solubility, salt type AB, Ksp = 1.8e-10.
Frequently asked questions
How do I calculate Ksp from molar solubility?
Write the ions produced per formula unit, replace each with a multiple of s, and multiply. AB salts: Ksp = s². AB₂ or A₂B: Ksp = 4s³. Enter s in the Ksp Calculator’s solubility → Ksp mode and pick the matching salt type.
How do I find Ksp or molar solubility the other way?
If you are given Ksp, invert the same relation: s = √Ksp for AB, s = (Ksp/4)^(1/3) for AB₂ / A₂B. Choose Ksp → solubility, enter the tabulated Ksp, and read s in mol/L.
Is this a precipitation reaction calculator?
For the Q vs Ksp test, yes. Enter the actual ion concentrations to get Q. Q > Ksp predicts a precipitate; Q < Ksp means no solid yet; Q = Ksp is saturated. It does not write a full net-ionic precipitation equation—use the Chemical Equation Balancer for that.
Does this include the common-ion effect?
The s ↔ Ksp modes assume pure water (no extra common ion). For common-ion problems, set up the ICE table with the extra ion and use the Chemical Equilibrium Calculator or solve algebraically; you can still check Q vs Ksp with measured ion concentrations.
Are Ksp values temperature-dependent?
Yes. Tabulated Ksp values are for a stated temperature (often 25 °C). Using a Ksp at the wrong temperature gives the wrong solubility.
References & further reading
Standards bodies, university open courseware, and peer-reviewed references that align with the methods used on this page.
See also: Chemical Equilibrium Calculator · Chemical Equation Balancer · pH Calculator
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